This invention relates to a signal processing apparatus, and more particularly to a signal processing apparatus for frequency-resolving or-decomposing an input signal based on the spectral sensitivity characteristic and deriving a high-definition signal by using the high-frequency component of a signal of a spectral sensitivity characteristic containing a large amount of information to estimate the high-frequency component of a signal of another spectral sensitivity characteristic containing a less amount of information.
Recently, image input devices which are inexpensive and light in weight and each use a single-plate type imaging device are widely used. In the single-plate type imaging device, in order to derive color information of a subject from one sheet of imaging device, color filters are arranged in a mosaic form on the light receiving surface. FIG. 18 shows the arrangement of a complementary color mosaic filter of cyan (Cy), magenta (Mg), yellow (Ye) and green (G) which is generally used.
In FIG. 18, luminance signals and color difference signals for the n-th line and (n+1)th line of an even field are respectively denoted by Y.sub.o,n, Y.sub.o,n+1 and C.sub.o,n, C.sub.o,n+1. Likewise, luminance signals and color difference signals for the n-th line and (n+1)th line of an odd field are respectively denoted by Y.sub.e,n, Y.sub.e,n+1 and C.sub.e,n, C.sub.e,n+1. In this case, the above signals can be expressed by the following equations. EQU Y.sub.o,n =Y.sub.o,n+1 =Y.sub.e,n C.sub.e,n+1 =2R+3G+2B (1) EQU C.sub.o,n =C.sub.e,n =2R-G (2) EQU C.sub.o,n+1 =C.sub.e,n+1 =2B-G (3)
Cy, Mg and Ye are expressed by the following equations by use of green (G), red (R) and blue (B). EQU Cy=G+B (4) EQU Mg=R+B (5) EQU Ye=R+G (6)
As indicated by the equation (1), the luminance signals are created on all of the lines in the even field and odd field. However, as indicated by the equations (2) and (3), the two color difference signals are created on every other lines, and lines on which color difference signals are not created are compensated for by interpolation. After this, three primary colors of R, G, B can be derived by performing the matrix operation. In the above method, the color signal has an information amount only half that of the luminance signal.
In Jap. Pat. Appln. KOKAI Publication No. 5-56446, there is disclosed a method for correcting the color difference signals by use of the luminance signal components instead of performing the simple interpolation by use of only the color difference signals as described before. That is, the luminance signal Y and the color difference signal C are processed by use of a low-pass filter to derive low-frequency components Y.sub.low and C.sub.low and a color signal C' obtained after correction is represented by the low-frequency components as indicated by the following equation. EQU C'=Y(C.sub.low /Y.sub.low) (7)
This means that the color difference signal C' after correction can be replaced by a signal obtained by correcting the luminance signal.
However, first, the above prior art technique only corrects the color signal by linear interpolation or by replacing the same by a signal obtained by correcting the luminance signal and cannot cope with correction of the color signal with high precision.
Second, the above prior art technique only corrects the color signal by interpolating the color signal by use of adjacent color signals or by replacing the same by a signal obtained by correcting the luminance signal on the same coordinate and cannot cope with a case where the continuity of the signal is degraded at the edge portion or the like or where the degree of correlation with the luminance signal is low.
Third, the above prior art technique only corrects the color signal by linear interpolation or by replacing the same by a signal obtained by correcting the luminance signal and cannot cope with a case where a high-frequency component of frequency higher than an input signal is created.
Fourth, the above prior art technique only corrects the color signal by linear interpolation or by replacing the same by a signal obtained by correcting the luminance signal and cannot cope with optimum correction for high- and low-frequency components.